The invention relates to filters for trapping solid particulates present in fluid streams and, in particular, to filters formed from thin porous walled, honeycomb structures.
The removal of solid particulates from fluids, gases or liquids, in which the particulates are mixed is typically accomplished by means of filters made from solid materials which are formed into articles or masses having a plurality of pores of small cross-sectional size extending therethrough, which may be interconnected, such that the solid materials are both permeable to the fluids which flow through the article or mass and capable of restraining most or all of the particulates mixed in the fluid from passing through the article or mass, as desired. Such pores constitute what is termed "open porosity" or "accessible porosity". The particulates are collected on the inlet surface(s) and/or within the pores of the material. The minimum cross-sectional size of some or all of the pores can be larger than the size of some or all of the particulates to be removed from the fluid, but only to the extent that significant or desired amounts of sufficiently large particulates become trapped on or within the filters during the transit of contaminated fluid. As the mass of collected particulates increases, the flow rate of the fluid through the filter generally decreases to an undesirable level. The filter is then either discarded as a disposable, replaceable element or regenerated by suitably removing the collected particulates so that it may be reused.
Certain considerations are helpful in evaluating and comparing filters. One is filter efficiency, the amount of the suspended particulates of concern that are removed from the fluid as it passes through the filter, usually expressed as a weight percentage of the total particulates of concern in an equal volume of fluid prior to its passing through the filter. Another consideration is flow rate, the volume of fluid per unit of time that passes through the filter and collected particulates. In a closed, continuous feed system, the consideration comparable to flow rate is pressure drop, that is the difference between the fluid pressure upstream and downstream from the filter caused by the presence of the filter and particulates thereon. Yet another consideration is operating time, the cumulative time of service of a filter before its flow rate/back pressure become unacceptable so as to necessitate replacement and/or regeneration of the filter. Lastly, there is capacity, the amount of particulate collected by the filter before an insufficient flow rate or excessive pressure drop is reached. Other desirable features of the filter include compact structure, mechanical integrity, chemical inertness or non-deleterious reaction with the fluid flow and particulate material.
It has been known for some time that honeycomb structures having a plurality of hollow passages or cells extending through them, which are formed in whole or in part by thin porous interconnected walls, may be successfully employed in filtering applications. U.S. Pat. Nos. 4,060,488, and 4,069,157 describe fluid filtering apparatus constructed by applying to a porous support body such as a thin porous walled honeycomb structure, a suitable filtering membrane. The membrane allows a portion of a fluid solution or mixture, which is transported through the honeycomb structure passages supporting the membrane, to pass into the porous support structure. The filtrate fluid migrates through the open porosity of the support structure to an internal reservoir or to an outer surface of the structure where it is removed.
U.S. Pat. Nos. 4,041,591 and 4,041,592, assigned to the assignee hereof, describe multiple flow path bodies and methods for fabricating the same from honeycomb structures having columns or layers of hollow cells which extend in a substantially mutually parallel fashion through the structure and which are formed by a plurality of thin intersecting walls. The structure is modified so that two separate fluids may be transported through alternate columns or layers of cells. It is suggested that the described multiflow path body might optionally be used in filtration and osmotic separation applications by using porous materials to produce the original honeycomb structure. As described, the device would function like those of the aforesaid U.S. Pat. Nos. 4,060,488, and 4,069,157 which allow only a fraction of a fluid which is passed through the alternate columns or layers of cells to migrate across the thin porous walls into the adjoining interleaved columns or layers of cells while the remainder of the fluid, having a higher concentration of contaminant or separable constituent, can continue to pass through and exit the structure.
Porous walled honeycomb structures may also be used directly to filter all or substantially all of the solid particulates from all fluid passed through the structure. Application Ser. No. 165,646, filed July 3, 1980 and assigned to the assignee hereof, and U.S. Pat. No. 4,276,071 both describe filters formed from honeycomb structures, themselves formed by matrices of thin porous intersecting walls, which define a pair of open, opposing end faces and a plurality of hollow, substantially mutually parallel passages or cells extending longitudinally through the structure between the end faces. At each end face where the open transverse areas of the cells would normally be exposed, the ends of alternate cells are blocked in a checkered or checkerboard pattern. The pattern is reversed at either end face so that each cell of the structure is blocked at only one end face and shares common thin walls and adjoining cells which are blocked only at the opposing end face of the structure. A contaminated fluid is introduced under pressure to a "inlet" end face of the filter body and passes into those cells open at the inlet end face (i.e. "inlet" cells). Because the inlet cells are closed at the opposing "outlet" end face of the structure, the contaminated fluid is forced to pass across the narrow dimension of the thin, porous walls into the adjoining "outlet" cells which are all blocked at the inlet end face and open at the outlet end face of the structure. Solid particulate matter which is sufficiently large is deposited on the thin wall surfaces defining the interior of the inlet cells or is trapped in the pores of the thin walls. All of the contaminated fluid passing through the structure is filtered and all of the thin walls of the structure are utilized in the described manner of filtering. U.S. Pat. No. 4,276,071 also describes a second crossflow-type honeycomb filter embodiment in which layes of longitudinally extending and laterally extending passages are alternately stacked. Diesel exhaust gas containing carbonaceous solid particulate contaminant is passed into either end of one of the two commonly extending sets of passages and is recovered after filtering from the remaining set of interleaved commonly extending passages. As only the thin walls separating the lateral and longitudinal layers of cells are porous, filtration occurs only through the thin walls shared in common between adjoining inlet and outlet passages. Accordingly, this device is only about one-half as efficient as a comparably sized and dimensioned honeycomb structure incorporating the previously described checkered pattern of alternate cells.